The present invention relates to apparatus for controlling refrigeration equipment, such as utilized in building air-conditioning systems; and particularly to controlling the speed of a fan which moves air across a condenser coil in those systems.
In the warmer months of the year, the environment inside a building is maintained at a desired temperature by an air conditioning system. Furthermore, buildings with windows that can not be opened may operate the air conditioning system even when it is relatively cold outside in order to dissipate heat from sources inside the building.
FIG. 1 illustrates the components of a refrigeration system 10 in which a refrigerant is circulated between a pair of heat exchangers located inside and outside the building in order to transfer heat from the building. The refrigerant exits a compressor 12 in the vapor phase at high temperature and pressure and flows through tubing of an air-cooled condenser 14. Air is forced over the condenser tubing by a fan 16 to transfer heat from the refrigerant to the exterior environment. As heat is removed, the refrigerant condenses into a liquid phase at moderate temperature and high pressure.
The liquid phase refrigerant then passes through an expansion device 15 which effects a pressure drop transforming the refrigerant into a mixed liquid and vapor phase at a much lower temperature and pressure. This liquid/vapor mixture flows into an evaporator 17 inside the building where the refrigerant is evaporated by interior air forced over the evaporator by another fan 18. Heat is removed from that flowing air thus cooling the interior of the building. The refrigerant leaves the evaporator 17 in a vapor phase at relatively low pressure and returns to the input of the compressor 12 where the refrigeration cycle repeats.
If the condenser air flow is too low an insufficient amount of heat will be removed from the refrigerant flowing through the condenser 14. If that happens, the pressure can rise too high and damage the system. When the outside air is too cold, the pressure at the condenser may drop considerably reducing the supply of liquid refrigerant to the evaporator 17 producing frost on the outer surface of the evaporator tubing.
To prevent these harmful conditions from occurring, various methods of pressure based control for air-cooled condensers have been developed. For that purpose, pressure sensors are provided at the condenser and the measured pressure is compared by a controller to a maximum acceptable absolute value. If the measured pressure exceeds the maximum value, the controller shuts off the compressor.
New refrigerants have been developed in response to environmental concerns regarding chlorofluorocarbon compounds. However, the more xe2x80x9cenvironmentally friendlyxe2x80x9d new refrigerants require that the refrigeration system operate at higher pressures than systems using chlorofluorocarbon refrigerants. As a consequence, newer refrigeration systems are more prone to leak at joints in the tubing which carry the refrigerant. Thus it is desirable to eliminate as many joints as possible.
Because pressure sensors require physical coupling to the tubing of the refrigeration system, system control based on temperature rather than pressure has been considered. Such systems would mount temperature sensors to the exterior surface of the condenser tubing. However, temperature based control responds relatively slowly to system changes. The tubing first has to change temperature in response to the refrigerant and then the sensor has to respond to that temperature change. Therefore, the system may operate in a potentially damaging mode for some time before the temperature sensor provides an indication of that situation.
The condenser fan in a refrigeration system has its speed controlled so that the air flow produced by the fan is commensurate with the ambient temperature at the condenser. That control involves sensing a temperature of the condenser to produce a temperature measurement.
The control method has a first state in which speed of the fan is varied in response to a comparison of the temperature measurement to a temperature set point. For example, when the temperature measurement is greater than the temperature set point, the fan speed is increased to move more air across the condenser to lower the refrigerant temperature. Similarly, the fan speed is decreased when the temperature measurement is less than the temperature set point and the temperature of the refrigerant needs to be raised.
The fan speed control has a second state which is utilized when the fan has been operated at a given relatively slow speed (e.g. the slowest possible constant speed) and the temperature measurement is less than the temperature set point. In this case, the air flow needs to be reduced to allow the condenser temperature to increase, but the fan may not be able to run any slower. In the second state, the fan is pulsed on and off to send bursts of air across the coil thus reducing the total air flow. The fan is controlled by a pulse width modulated signal having a duty cycle that is varied in response to the temperature measurement.
A transition occurs from the first state to the second state when a first predefined condition occurs. For example this condition may be when the fan has operated at the slowest possible speed for a predetermined period of time while the temperature measurement is below the temperature set point. A transition occurs from the second state to the first state upon the occurrence of a second predefined condition, such as when the duty cycle of the fan is at 100% and the temperature measurement is above the temperature set point, for example.
Another aspect of the present invention is a unique start-up state in which the control is configured during a period in which the refrigeration system is warming-up. That warm-up occurs differently depending upon the ambient temperature of the environment in which the condenser is located.